This invention relates to systems for converting the frequency band of electrical picture information signals, and more particularly to a system of the kind above described which is suitable for the transmission or recording and reproduction of a still picture or a frame of a television signal by means of a transmission medium having a narrow frequency band characteristic such as a telephone line or audio magnetic tape.
There are various conventional methods for converting the frequency band of an electrical picture information signal. A first method comprises employing a rotary memory such as a magnetic disk, magnetic sheet, magnetic drum or endless magnetic tape having a capacity of storing one frame portion of electrical picture information signal and varying the rotating speed of the rotary memory depending on the writing operation and reading operation. A second method comprises employing a stationary memory such as a magnetic core memory, magnetic wire memory, semiconductor shift register memory or capacitor memory having a capacity of storing one frame portion of electrical picture information signal and varying the frequency of clock pulses depending on the writing operation and reading operation. A third method comprises employing a memory of the electron beam scanning type such as a charge storage tube memory having a capacity of storing one frame portion of electrical picture information signal and varying the scanning rate depending on the writing operation and reading operation. However, these conventional methods have the following defects: The first method employing the rotary memory is defective in the lack of quick responsiveness due to the fact that an elongated stabilizing period of time is required during change-over of the rotating speed. The second method employing the stationary memory is defective in that the memory is more expensive than those in any other methods and thus this method is not economical. The third method employing the memory of the electron beam scanning type is defective in that satisfactory fidelity for the transmitted signal is difficult to obtain.